Method for joining electrical conductors by magnetostriction and magnetostriction-generating device

ABSTRACT

The invention relates to a method for the joining, to a first conductor that includes a connector, of a second conductor formed by at least one strand made of aluminium or an aluminium alloy and having at least one end, the shape of which allows it to be introduced into the connector of the first conductor. The method has a step of installing the conductors, with the end of the second conductor being inserted into the connector of the first conductor. There is a magnetostriction step which has an electrodynamic force being generated in the connector so that the connector is crushed around the second conductor introduced into the connector. The subject of the invention is also a magnetostriction-generating device. In particular, the invention applies to the connecting-up of energy networks on all (for example, aerospace) carriers and equipment, with high-current and/or long links. The invention may also apply to connections suitable for information transmission on general-purpose conductors.

RELATED APPLICATIONS

The present application is based on, and claims priority from, FrenchApplication Number 06 11261, filed Dec. 22, 2006, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to a method for joining electrical conductors bymagnetostriction and to a magnetostriction-generating device. Inparticular, the invention applies to the connecting-up of energynetworks on all (for example aerospace) carriers and equipment, withhigh-current and/or long links. The invention may also apply toconnections suitable for information transmission on general-purposeconductors.

BACKGROUND OF THE INVENTION

The use of power cables comprising strands made of aluminium or analuminium alloy has many advantages, especially a significant saving forsome applications in terms of weight compared with copper strands. Thereare for example cables with a so-called sectorial solid core, consistingof three phase conductors, in the form of 120° circular sectors, and alayer of small circular conductors constituting the neutral. Althoughthe manufacture of this type of cable proves to be more expensive, theeconomic gain is real when the laying and transport costs are taken intoaccount.

It is known to employ on such cables bimetallic end-fittings crimped attheir aluminium end. They are then joined to copper cables and otherequipment via the copper side of said end-fittings. There are severaltypes of end-fittings, suitable for each case (mid-line tapping,terminations, end-to-end connection, repair, etc.). The joint betweenthe copper part and the aluminium part of the end-fitting is ametallurgical joint, in particular by welding. The joining procedure isthe following:

-   -   after stripping, a tool is used to deform a 120° sector zone        into a circular cylinder, the surface alumina layer is removed        by vigorous brushing (for example with a metal brush) under        neutral grease, this solution being compatible only with a        conductor sufficiently solid to withstand such a treatment;    -   once the end-fitting has been slipped onto the conductor, the        assembly is subjected to a deep punching operation (penetration        of the metal of the end-fitting into the core of the metal of        the conductor), creating an air-tight contact.

Although this procedure proves to be satisfactory for large cables, itis not suitable for all cables. In particular, this procedure isill-suited to multistrand cables. Now, connections on aeronautical oraerospace carriers, in which the criterion of withstanding vibrations isparticularly key, require such multistrand cables, especially for theirflexibility characteristic.

SUMMARY OF THE INVENTION

One object of the invention is in particular to alleviate theaforementioned drawbacks. For this purpose, one subject of the inventionis a method for the joining, to a first conductor that includes aconnector, of a second conductor formed by at least one strand made ofaluminium or an aluminium alloy and having at least one end, the shapeof which allows it to be introduced into the connector of the firstconductor. The method comprises at least the following steps:

-   -   a step of installing the conductors, the end of the second        conductor being inserted into the connector of the first        conductor; and    -   a magnetostriction step, an electrodynamic force being generated        in the connector so that said connector is crushed around the        second conductor introduced into the connector.        Furthermore, the magnetostriction step may comprise:    -   a substep for heating the second conductor and the connector        until their plastic deformation point is reached;    -   a substep for thermally stabilizing the second conductor and the        connector; and    -   a substep for implementing the electrodynamic effect of the        magnetostriction.

The method may also include a step of preparing the two conductors,comprising:

-   -   a substep in which the second conductor is stripped at its end;    -   a substep in which the stripped part of the second conductor and        at least the internal part of the connector of the first        conductor are pickled; and    -   a substep in which the stripped part of the second conductor and        at least the internal part of the connector are rinsed and then        dried.

Advantageously, the step of preparing the two conductors and/or the stepof installing the conductors may be carried out in a flow of inert gas.

Another subject of the invention is a magnetostriction-generating devicesuitable for joining, to a first conductor that includes a connector, asecond conductor formed by at least one strand made of aluminium or analuminium alloy and having at least one end, the shape of which allowsit to be introduced into the connector of the first conductor. Thedevice comprises:

-   -   an induction loop generating a magnetic field, the induction        loop being designed to couple said magnetic field to the        deformable part of the connector, the shape and the dimensions        of the loop being suitable for holding the deformable part in        place;    -   an inductor connected via a fourth switch to a voltage        generator, to a current generator and to a third switch which        makes it possible to select whether the inductor is connected to        the current generator or to the voltage generator;    -   an AC current generator coupled to the inductor and to the        induction loop via a transformer, a first switch being placed in        parallel with the AC current generator; and    -   a circuit comprising a second switch and a capacitor in parallel        with the inductor.

Advantageously, the induction loop and/or the transformer and/or theinductor may be cooled.

The device may especially be included in a coaxial toroidal envelope,said outer envelope contributing to reducing the magnetic fields due tothe induction and to the transformer through the Frager turn effect.

The device may be used in the following operating sequence:

-   -   the first switch is opened;    -   the second switch is closed;    -   position on the third switch is selected, making it possible to        supply the inductor by the voltage generator until sufficient        energy for implementing the electrodynamic effect is stored in        the inductor;    -   the AC current generator is implemented so that sufficient        thermal energy to ensure the correct temperature has been        transferred into the connector;    -   the AC current generator is employed for the time needed for the        heat to diffuse into the conductor placed in the connector and        position on the third switch is selected, making it possible to        supply the inductor by the current generator during this time;    -   the second switch is opened and the first switch is closed; and    -   the fourth switch is opened.

The invention has in particular the advantages that it does not requirethe conductors to undergo a particular surface treatment: simpleuncoated aluminium multistrand conductors are suitable. The inventionmakes it possible to reduce the production costs and to improve thequality of the joint. The connectors employed are not necessarilybimetallic.

Still other advantages of embodiments according to the present inventionwill become readily apparent to those skilled in the art from thefollowing detailed description, wherein the preferred embodiments of theinvention are shown and described, simply by way of illustration of thebest mode contemplated of carrying out the invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modifications in various obviousrespects, all without departing from the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not bylimitation, in the figures of the accompanying drawings, whereinelements having the same reference numeral designations represent likeelements throughout and wherein:

FIG. 1, by means of a block diagram, a method according to the inventionfor joining electrical conductors by magnetostriction;

FIG. 2, by means of a block diagram, the various substeps of themagnetostriction step of the method according to the invention;

FIG. 3, by means of a block diagram, the various substeps of the step ofpreparing the conductors of the method according to the invention; and

FIG. 4, by means of a diagram, a magnetostriction-generating deviceaccording to the invention.

FIG. 1 shows, by means of a block diagram, a method according to theinvention for joining electrical conductors by magnetostriction.

Magnetostriction is a magnetic-pulse cold-welding technique. For this, acoil, in which an electric current flows, is used to create acentripetal force in an annular connector electromagnetically coupled tosaid coil. A sudden change in the current flowing in the coil generatesa large electro-dynamic force on the annular connector, theelectrodynamic force being proportional to the derivative of the currentwith respect to time. Furthermore, the electric current produces, byinduction, a thermal effect on the connector.

The method according to the invention makes it possible in particular tojoin two conductors, at least one of which is a conductor made ofaluminium or an aluminium alloy. The conductors may be conductors with amultistrand core made of untreated aluminium alloy. The number ofstrands is defined so that the conductors achieve the desired mechanicalproperties, for example in terms of flexibility. The strands may betwisted (in particular so as to obtain better cohesion) or they may bestraight (the strands may then have the form of a 120° angular sector).In particular, the conductors may meet the insulation requirementsrequired in the aeronautical field.

The first of the two conductors includes an annular connector. Forexample, the annular connector is a connection end-fitting of theconnector pin type, terminated by an approximately tubular cavity. Thecavity may be made of aluminium, an aluminium alloy or any otherconducting material suitable for magnetostriction. The approximatelytubular cavity of the first conductor may be made of a material that isductile at a defined temperature, having an expansion coefficientsubstantially the same or higher than the metal making up the firstconductor. The latter feature makes it possible in particular duringcooling to clamp the first conductor around the second. The dimensionsand the shape of the cavity are matched to those of the first conductor,in particular in the case in which the conductors do not have asubstantially circular shape. The thickness of the annular connector isespecially defined so as to reconcile its ductility duringimplementation of the magnetostriction, its mechanical strength and itscapability of being inserted into an insulator. The second conductor hasat least one end, the shape of which allows it to be introduced into theconnector of the first conductor.

The method according to the invention optionally includes a step 1 ofpreparing the two conductors. Shown in the block diagram of FIG. 3 arevarious substeps of step 1 of preparing the two conductors. Thus, in asubstep 10, if necessary the second conductor is stripped at its end soas to expose the strands of which it is composed. Next, in a substep 11,the stripped part of the second conductor and at least the internal partof the connector of the first conductor are pickled. For this, it ispossible to employ a triacid to remove the alumina from the variousparts. Next, in a substep 12, the stripped part of the second conductorand at least the internal part of the connector of the first conductorare rinsed. The rinsing operation may be carried out with pure water.Substep 12 is completed by a drying operation. The drying may be carriedout using a flow of inert gas, such as argon, or a reducing gas,optionally heated.

The method according to the invention includes a step 2 of installingthe conductors. The end of the second conductor is inserted into theconnector of the first conductor. The assembly formed by the twoconductors thus positioned is introduced into themagnetostriction-generating device. Alternatively, the annular connectorof the first conductor is introduced into themagnetostriction-generating device before the end of the secondconductor is inserted into the annular connector. To increase thereliability, these operations may be carried out under a flow of inertgas, so that the inside of the annular connector is in contact withoxygen as little as possible. This operating method reduces the risk ofcreating an alumina film when the connector undergoes a temperature riseduring the magnetostriction.

The method according to the invention includes a magnetostriction step3. An electrodynamic force is generated by themagnetostriction-generating device in the annular connector. The forceproduced on the annular connector by the magnetostriction crushes theannular conductor around the second conductor introduced into theannular connector. This technique makes it possible to obtain ametallurgical weld between the conducting strands and the annularconnector with the external layer of the conducting strands, theassembly being airtight. Shown in the block diagram of FIG. 2 arevarious substeps of magnetostriction step 3. Magnetostriction step 3includes a first heating substep 31. Heating substep 31 may be carriedout by induction heating. In heating substep 31, the second conductorand the connector are heated up to their plastic deformation point. Theplastic deformation increases the reliability of the joint avoiding therisk of a crack or crack initiator appearing. The curve representing therise in temperature is a relatively slow ramp compared with othersequences carried out during magnetostriction, on which a high-frequencyAC signal may be superimposed, in particular to improve the inductionheating. Magnetostriction step 3 includes a thermal stabilization secondsubstep 32. Thermal stabilization substep 32 allows the thermal wave toreach the surface of the conducting strands. The curve showing the risein temperature then has an approximately constant plateau, on which ahigh-frequency AC signal may be superimposed. Magnetostriction step 3includes a third substep 33, for implementing the electrodynamic forceof the magnetostriction. Substep 33 has a maximum duration of a few tensof milliseconds. If necessary, substep 33 is followed by a cooling step,which may lead to work-hardening of the connector, optionallysupplemented by an annealing operation.

FIG. 4 illustrates by means of a diagram a magnetostriction-generatingdevice according to the invention. The elements identical to theelements already presented in the other figures bear the samereferences. The device is especially suitable for implementingmagnetostriction step 3 of the method according to the invention.

The device includes an induction loop 100 generating the magnetic field.The induction loop 100 couples said magnetic field to the deformablepart 102 of a connector, the stripped part of a conductor 110 (theconductor 100 moreover being within an insulator 109) having beenintroduced into the deformable part 102. The shape and the dimensions ofthe loop 100 are furthermore suitable for holding the deformable part102 in place. The deformable part 102 may be pinched, for example bymeans of an opening loop, or else may be slipped thereinto. Theinduction loop 100 may be cooled, especially by a coolant 101. Thecooling operation makes it possible both to guarantee the mechanicalintegrity of the induction loop 100 and a temperature that can bewithstood by the operator handling the device.

The device includes an inductor 103 connected to a ramp generator 106.The ramp generator 106 is shown in FIG. 4, especially for betterunderstanding, by a voltage generator 104 and a current generator 105,and a third switch K3 for selecting one or other of them. However, theswitched ramp generator 106 may consist of a single generator providedwith two operating modes. It may furthermore include a dynamicprotection device, protecting it against external overvoltages, forexample by means of a cut-off voltage regulator. The inductor 103 isconnected via a fourth switch K4 to the ramp generator 106.

The inductor 103 can therefore be connected to the voltage generator 104or to the current generator 105 via the third switch K3 which makes itpossible to select whether the inductor 103 is connected to the currentgenerator 105 (with the third switch K3 in position a in FIG. 4) or tothe voltage generator 104 (with the third switch K3 in position b inFIG. 4). When the inductor 103 is connected to the voltage generator104, it stores the energy delivered by the voltage generator 104. Thisis because a current delivered by the voltage generator 104 flowsthrough the inductor 103, which current increases linearly with time.The energy thus stored increases quadratically with time. When theinductor 103 is connected to the current generator 105 delivering aconstant current, the inductor 103 maintains the energy that it hasstored previously.

When the inductor 103 is short-circuited by means of the fourth switchK4, it restores the energy that it has stored, causing a sudden changein the current. To maximize the change in current, especially tomaximize the electrodynamic force on the induction loop 100, the ohmicresistance of the circuit must be as low as possible. In particular, theinductor 103 must have the lowest possible ohmic resistance. However, inorder for the energy stored within the inductor 103 to be maximized, itis necessary for the inductance of the inductor 103 to be as high aspossible: the inductor 103 must therefore have the largest number ofturns possible. To reconcile these two contradictory requirements, it ispossible for example to cool, or even refrigerate, the inductor 103. Theinductor 103 will for example be of the toroidal type and/or coiled on amagnetic circuit, both to maximize the inductance and to minimize themagnetic losses.

The fourth switch K4 is subject to large electrical, mechanical andthermal stresses, and consequently must be designed accordingly.Furthermore, an aid to rapid switching may be added to the fourth switchK4. For example, an electromechanical circuit breaker of the magnetic orgas blast type may meet these requirements.

The device includes an AC current generator 111. The AC currentgenerator delivers power intended for heating the deformable part 102 ofthe connector. The AC current generator 111 is coupled to the inductor103 and to the induction loop via a transformer 108. A first switch K1is placed in parallel with the AC current generator 111 so as to be ableto short-circuit the transformer 108 to the primary or to the secondary,depending on the relative dimensions of the elements. The first switchK1 makes it possible in particular to protect the AC current generator111 from the current pulse during the magnetostriction. The magneticcircuit of the transformer 108 may be saturable (by decoupling betweenthe primary and the secondary caused by the saturation resulting fromthe passage of the pulse generated during the magnetostriction),improving the protection of the AC current generator 111 at the cost ofa slight loss of energy. In particular, the transformer 108 must havethe lowest possible ohmic resistance. It is possible for example tocool, or even refrigerate, the transformer 108. The AC current generator111 is adjustable as regards at least two power levels (one levelsuitable for heating and one level suitable for maintaining temperature)and for values that vary according to the type of the deformable part102 of the connector.

The inductor 103 forms a parallel circuit with a second switch K2 and acapacitor 107. The capacitor 107 makes it possible in particular toshort-circuit the inductor 103 when the AC current generator 111 is usedfor heating the deformable part 102. When the inductor 103 storesenergy, the spectral content is too low for the current in the inductor103 to be stored in the capacitor 107.

The connection method employed by the device according to the inventionmust be optimized in terms of radiation and electrical resistivity. Inone embodiment, one method of eliminating the parasitic inductanceinduced in the circuit is to duplicate each connection segment by itsidentical copy, while reversing the direction of the current therein. Inanother embodiment, the connection is produced using a coaxial cableconsisting of two tubular conductors, which is cooled by circulating acoolant. Such an embodiment may also apply to the inductor 103, to thetransformer 108, to the fourth switch K4 and to the ramp generator 106.

The device according to the invention may be constructed so as to beincluded in a coaxial structure. Thus, the device according to theinvention may be included in a coaxial toroidal envelope. Said outerenvelope may contribute to reducing the magnetic fields due to theinductor 103 and to the transformer 108, through the Frager turn effect.

The operating sequence of the magnetostriction-generating deviceaccording to the invention is the following:

-   -   initially:        -   the first switch K1 is opened and the AC current generator            111 is then connected to the circuit; and        -   the second switch K2 is closed and the capacitor 107 is then            charging.    -   in a current-rise/heating phase corresponding in particular to        substep 31 of the method according to the invention:        -   the inductor 103 is supplied by the voltage generator 104            (with the third switch K3 in position a), a current flowing            through the inductor that increases linearly until            sufficient energy for implementing the electrodynamic effect            (substep 33 of the method according to the invention) is            stored in the inductor 103 (the time having moreover to be            chosen to be long enough to prevent too large a current            drift resulting in premature deformation of the connector            102); and        -   the AC current generator 111 delivers power so that, at the            end of the current rise, sufficient thermal energy for            ensuring the correct temperature has been transferred into            the end-fitting 102;    -   in a thermal-diffusion/current-maintaining phase corresponding        in particular to substep 32 of the method according to the        invention, the energy needed for the correct temperature being        in the end-fitting 102 but poorly distributed (too close to the        surface because of the skin effect),        -   the AC current generator 111 delivers, during the time            needed for the heat to diffuse into the conductor placed in            the connector 102, the power needed to maintain the            temperature of the connector 102, designed to compensate for            the thermal losses; and        -   the current generator 105 supplies the inductor 103 with            constant current (position b of the third switch K3) so as            to compensate for the losses due to the ohmic resistance of            the circuit;    -   the AC current generator 111 is protected by:        -   turning the AC current generator 111 off;        -   opening the second switch K2 in order to protect the            capacitor 107 and prevent it from diverting some of the            energy intended for the induction loop 100; and        -   closing the first switch K1 in order to protect the AC            current generator 111;    -   in a crimping phase corresponding in particular to substep 33 of        the method according to the invention, the fourth switch K4 is        opened so as to break the current in the inductor 103 as        suddenly as possible; and    -   in an optional phase of cooling and annealing the connector 102,        corresponding to step 4 of the method according to the        invention, the fourth switch K4 remains open.

It will be readily seen by one of ordinary skill in the art thatembodiments according to the present invention fulfill many of theadvantages set forth above. After reading the foregoing specification,one of ordinary skill will be able to affect various changes,substitutions of equivalents and various other aspects of the inventionas broadly disclosed herein. It is therefore intended that theprotection granted hereon be limited only by the definition contained inthe appended claims and equivalents thereof.

1. A method for the joining, to a first conductor that includes aconnector, a second conductor formed by a strand made of aluminium or analuminium alloy and having at least one end, the shape of which allowsit to be introduced into the connector of the first conductor,comprising the following steps: a step of installing the conductors, theend of the second conductor being inserted into the connector of thefirst conductor; and a magnetostriction step, an electrodynamic forcebeing generated in the connector so that said connector is crushedaround the second conductor introduced into the connector.
 2. The methodaccording to claim 1, wherein said magnetostriction step comprises: asubstep for heating the second conductor and the connector until theirplastic deformation point is reached; a substep for thermallystabilizing the second conductor and the connector; and a substep forimplementing the electrodynamic effect of the magnetostriction.
 3. Themethod according to claim 1, including a step of preparing the twoconductors, comprising: a substep in which the second conductor isstripped at its end; a substep in which the stripped part of the secondconductor and at least the internal part of the connector of the firstconductor are pickled; and a substep in which the stripped part of thesecond conductor and at least the internal part of the connector arerinsed and then dried.
 4. The method according to claim 1, wherein saidstep of preparing the two conductors and/or step of installing theconductors are carried out in a flow of inert gas.
 5. Amagnetostriction-generating device suitable for joining, to a firstconductor that includes a connector, a second conductor formed a strandmade of aluminium or an aluminium alloy and having at least one end, theshape of which allows it to be introduced into the connector of thefirst conductor, comprising: an induction loop generating a magneticfield, the induction loop being designed to couple said magnetic fieldto the deformable part of the connector, the shape and the dimensions ofthe loop being suitable for holding the deformable part in place; aninductor connected via a fourth switch to a voltage generator, to acurrent generator and to a third switch which makes it possible toselect whether the inductor is connected to the current generator or tothe voltage generator; an AC current generator coupled to the inductorand to the induction loop via a transformer, a first switch being placedin parallel with the AC current generator; and a circuit comprising asecond switch and a capacitor in parallel with the inductor.
 6. Thedevice according to claim 5, wherein the induction loop and/or thetransformer and/or the inductor are cooled.
 7. The device according toclaim 5, wherein said device is included in a coaxial toroidal envelope,said outer envelope contributing to reducing the magnetic fields due tothe inductor and to the transformer through the Frager turn effect. 8.The use of the device according to claim 5, in the following operatingsequence: the first switch is opened; the second switch is closed;position (a) on the third switch is selected, making it possible tosupply the inductor by the voltage generator until sufficient energy forimplementing the electrodynamic effect is stored in the inductor; the ACcurrent generator is implemented so that sufficient thermal energy toensure the correct temperature has been transferred into the connector;the AC current generator is employed for the time needed for the heat todiffuse into the conductor placed in the connector and position (b) onthe third switch is selected, making it possible to supply the inductorby the current generator during this time; the second switch is openedand the first switch is closed; and the fourth switch is opened.
 9. Themethod according to claim 2, including a step of preparing the twoconductors, comprising: a substep in which the second conductor isstripped at its end; a substep in which the stripped part of the secondconductor and at least the internal part of the connector of the firstconductor are pickled; and a substep in which the stripped part of thesecond conductor and at least the internal part of the connector arerinsed and then dried.
 10. The method according to claim 2, wherein saidstep of preparing the two conductors and/or step of installing theconductors are carried out in a flow of inert gas.
 11. The methodaccording to claim 3, wherein said step of preparing the two conductorsand/or step of installing the conductors are carried out in a flow ofinert gas.
 12. The device according to claim 6, wherein said device isincluded in a coaxial toroidal envelope, said outer envelopecontributing to reducing the magnetic fields due to the inductor and tothe transformer through the Frager turn effect.
 13. The use of thedevice according to claim 6, in the following operating sequence: thefirst switch is opened; the second switch is closed; position (a) on thethird switch is selected, making it possible to supply the inductor bythe voltage generator until sufficient energy for implementing theelectrodynamic effect is stored in the inductor; the AC currentgenerator is implemented so that sufficient thermal energy to ensure thecorrect temperature has been transferred into the connector; the ACcurrent generator is employed for the time needed for the heat todiffuse into the conductor placed in the connector and position (b) onthe third switch is selected, making it possible to supply the inductorby the current generator during this time; the second switch is openedand the first switch is closed; and the fourth switch is opened.
 14. Theuse of the device according to claim 7, in the following operatingsequence: the first switch is opened; the second switch is closed;position (a) on the third switch is selected, making it possible tosupply the inductor by the voltage generator until sufficient energy forimplementing the electrodynamic effect is stored in the inductor; the ACcurrent generator is implemented so that sufficient thermal energy toensure the correct temperature has been transferred into the connector;the AC current generator is employed for the time needed for the heat todiffuse into the conductor placed in the connector and position (b) onthe third switch is selected, making it possible to supply the inductorby the current generator during this time; the second switch is openedand the first switch is closed; and the fourth switch is opened.